A number of devices have heretofore been proposed and made commercially available for indicating point-level of material in a storage container or vessel. For example, U.S. Pat. No. 3,834,235 discloses a device in which an optical probe is positioned at the desired height of material level detection in the vessel. Light energy is directed into the probe from externally of the vessel. When material is spaced from the probe within the vessel, the difference in indices of refraction between the probe and air at the probe tip is such that the light energy is reflected internally of the probe for detection. On the other hand, when material covers the probe tip, the refractive index differential at the probe tip is so altered that the light energy escapes into the material. Hence, the fact that material has reached the level of the probe tip may be detected externally of the vessel by the absence of light energy internally reflected from the probe tip.
U.S. Pat. No. 3,625,058 discloses a device of this character in which a tuning fork is positioned at a predetermined height within the vessel at which material level indication is desired, and is coupled to a motor for vibrating the fork. When material is spaced from the fork, the fork is free to vibrate. However, when the material reaches the level of the fork and covers the fork, vibration is damped. Thus, the fact that the level of material in the vessel has reached the height of the fork may be detected as a function of vibration characteristics of the fork itself. U.S. Pat. No. 4,392,032 discloses a device of the subject character in which the probe takes the form of a paddle positioned within the vessel at the desired height of material level detection, and is connected to a motor carried within a suitable housing externally of the vessel. When the material is spaced from the paddle, the paddle is free to rotate as driven by the motor. However, when the material reaches the level of the paddle, paddle rotation is retarded, and such retardation may be detected externally of the vessel.
U.S. Pat. No. 4,499,766 discloses a material level indicating device in which a capacitance probe is positioned within a vessel such that electrical characteristics at the probe vary as a function of dielectric properties of the material, which in turn vary as a function of material level. The probe is connected in an LC resonant circuit to an rf oscillator, and phase shift of the probe signal is monitored to indicate changes in material level.
It is often desirable in systems of this character to provide indications of two or more material levels, in one or a number of vessels, as interrelated functions of each other. For example, it may be desirable to provide a system in which two probes are coupled to a vessel so as to be responsive to high and low levels of material within the vessel respectively. A system of this character may be employed for controlled filling of the vessel until the material covers both probes, and thereafter for controlled emptying of the vessel until both probes are uncovered. A general object of the present invention is to provide a system of such a character that is economical to manufacture, and that may be readily adjusted in the field by relatively unskilled personnel for providing indications of material level at the respective probes either independently of each other, as conjoint functions of each other or a combination thereof. Yet another object of the present invention is to provide a system of the described character that may be readily adjusted in the field for unusual conditions at the probe and/or for specific operating characteristics of the material in connection with which the system is to be used.